1. Field of the Invention
This invention relates generally to fuel cells, and, more particularly, to fuel cells that are conformable into desired shapes and that can be incorporated into the outer wall of a product or clothing.
2. Background Information
Fuel cells are devices in which electrochemical reactions are used to generate electricity from fuel and oxygen. A variety of materials may be suited for use as a fuel depending upon factors such as fuel availability and portability. Carbonaceous materials, such as methanol or natural gas, are attractive fuel choices due to their high specific energy.
Fuel cell systems that operate on carbonaceous fuels may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell system) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external processing upstream the fuel cell. Because fuel processing generally requires complex and expensive components, which occupy significant volume, reformer-based systems are presently limited to comparatively large, high power applications. Other, “direct hydrogen” systems require that pure hydrogen gas be fed to the anode of a fuel cell system, limiting their application.
Direct oxidation fuel cell systems using liquid fuel are better suited for a number of applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger applications. In the direct oxidation fuel cells of interest here, i.e. those that use a polymer electrolytes, a carbonaceous liquid fuel (typically methanol or an aqueous methanol solution) is introduced to the anode face of a membrane electrode assembly (MEA).
One example of a direct oxidation fuel cell system is a direct methanol fuel cell system or DMFC system. In a DMFC system, a mixture comprised of predominantly methanol or methanol and water is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. The fundamental reactions are the anodic oxidation of the fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water.
Typical DMFC systems include a fuel source, fluid and effluent management systems, and air management systems, as well as a direct methanol fuel cell (“fuel cell”) stack, or array, consisting of single cells connected electrically in series. The fuel cell stack, or array , typically consists of a housing , hardware for current collection, fuel and air distribution, and a number of membrane electrode assemblies (“MEAs”) disposed within the housing.
The electricity generating reactions and the current collection in polymer electrolyte direct oxidation fuel cell systems generally take place within the MEA. In the carbonaceous fuel oxidation process at the anode, the products are protons, electrons and carbon dioxide. Protons (from hydrogen atoms in the fuel and in water molecules involved in the anodic reaction) are separated from the electrons. The protons migrate through the membrane electrolyte, which is non-conductive to the electrons. The electrons travel through an external circuit, which connects the cell to the load where power is utilized, and are united with the protons and oxygen molecules in the cathodic reaction.
A typical MEA includes an anode catalyst layer and a cathode catalyst layer sandwiching a centrally disposed protonically-conductive, electronically non-conductive membrane (“PCM”, sometimes also referred to herein as “the catalyzed membrane”). One example of a commercially available PCM is NAFION® (NAFION® a registered trademark of E.I. Dupont de Nemours and Company), a cation exchange membrane based on polyperfluorosulfonic acid, in a variety of thicknesses and equivalent weights. The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. A PCM that is optimal for fuel cell applications possesses a good protonic conductivity and is well-hydrated in the operating cell. On either face of the catalyst coated PCM, the MEA typically includes a diffusion layer. The diffusion layer on the anode side is employed to evenly distribute the liquid or gaseous fuel over the catalyzed anode face of the PCM, while allowing the reaction products, typically gaseous carbon dioxide, to move away from the anode face of the PCM. In the case of the cathode side, a diffusion layer is used to allow a sufficient supply of and a more uniform distribution of gaseous oxygen to the cathode face of the PCM, while minimizing or eliminating the accumulation of liquid, typically water, on the cathode aspect of the PCM. Each of the anode and cathode diffusion layers also assist in the collection and conduction of electric current from the catalyzed PCM through to the load. Further details of the operation of a direct oxidation fuel cell and a discussion of fuel substances including a gel-based carbonaceous fuel substance are discussed in detail in commonly-owned U.S. Pat. No.: 7,255,947 issued on Aug. 14, 2007 by Juan J. Becerra et al. for a FUEL SUBSTANCE AND ASSOCIATED CARTRIDGE FOR FUEL CELL, filed Oct. 17, 2003, which is incorporated herein by reference.
Direct oxidation fuel cells are particularly suited for use with small portable electronic devices based on the sufficiency of such fuel cells' power output and the ability to manufacture the comparatively simple direct oxidation fuel cell system on a micro-level. Although certain non-planar designs have been suggested, as are noted hereinafter, it has not been heretofore known, however, to manufacture a fuel cell system that is configured to substantially conform to a predetermined non-planar shape, or which is disposed on a pliable substrate, and is therefore a pliable assembly. More specifically, it is desirable in certain applications to incorporate a fuel cell into an article of clothing (such as a belt or vest) or a surface of an application device in such a manner that the fuel cell incorporated into those items thus powers devices being used by the individual, such as telephones, personal digital assistants, other communication devices, GPS positioning and location devices, tracking devices, beepers, weaponry, listening aides and other equipment of an electronic nature that may be used, for example, by a soldier, law enforcement officer, security personnel or a person in an industry in which it is desirable to wear or employ a number of electronic devices on one's person, each of which require power. In such instances, it may be inconvenient to carry batteries or replacement batteries for each individual device. More importantly, the energy density of known batteries is not typically sufficient to allow an acceptable operating duration given their weight and volume characteristics.
It has been described how flexibility could be provided to the current collector of a fuel cell, which would allow the cell to be formed into certain non-planar shapes, including cylinders. However, it is important for maintaining optimum fuel cell performance to not just introduce non-planar or flexible components, but to maintain sufficient compression along the active surface area of the fuel cell. This is particularly important in a fuel cell that utilizes a polymer electrolyte without an additional liquid electrolyte, where the fuel cell typically cannot reliably generate power without sufficient compression (typically 100 psi or greater) over the active area that guarantees good current collector/MEA contact. Attempts to develop non-planar fuel cells that have been described do not appear to provide for adequate compression for the operation of a fuel cell. See, e.g. U.S. Pat. No. 6,620,542. Furthermore, incorporation of a fuel cell into an article of clothing, such as a vest, or into a fabric, that can then be sewn or otherwise attached to another article of clothing or a device, has not been considered.
As used herein, when used to describe a fuel cell, a fuel cell array or a fuel cell system, “conformable” shall mean being fabricated in such a fashion as to generally conform to the contours of the desired application or being sufficiently pliable to allow the assembly to meet a variety of shapes or to change shape based on the form of the object to which it is attached. There remains a need, therefore, for a viable conformable fuel cell that can be formed in a desired shape, including curved fuel cells and multifaceted fuel cells which can then be worn by an individual, or which can be incorporated into the fabric of an article of clothing or itself can comprise the whole article of clothing, or a panel thereof, or incorporated into a device, to supply power to devices being utilized by that individual.
It is thus an object of the present invention to provide a well-performing, conformable fuel cell that can either be formed in a desired shape that conforms to a particular body segment or location, or an application device, or which can be incorporated into an article of clothing within the fabric used for such clothing, or as a pliable fuel cell to be coupled to a device.